In self-assembled electrochemical devices, electrodes (anode and cathode) are formed from particles dispersed in an ionically conductive electrolyte phase. In the electrode structure, particles of like type contact one another due to attractive forces, while particles of unlike type are separated from one another due to repulsive forces. The magnitude of separation between unlike particles is generally at least some tens of nanometers to prevent electronic shorting of the device. An electrolyte typically occupies the area of separation between unlike particles.
The balance of attraction and repulsion between two types of particles in a self-assembled electrochemical device can be achieved via London dispersion forces: like particles generally attract one another when dispersed in a liquid medium, while unlike particles repel one another if the respective refractive indices of the first type of particle, the intervening liquid, and the second type of particle increase or decrease monotonically. Typical anode and cathode materials used in lithium secondary batteries (e.g., carbon and LixCoO2 (LCO), respectively) generally both have higher refractive indices than typical electrolyte materials. Accordingly, a monotonic change in refractive index in a self-assembled electrochemical device can be achieved by coating one type of electrode particle (anode or cathode) with an encapsulant material that has a refractive index below that of the electrolyte phase, which in turn has a refractive index below that of the other type of electrode particle. Often, the encapsulant material is electronically and ionically conductive, thus providing enhanced performance of the self-assembled electrochemical device, e.g., battery charge and discharge. For example, encapsulant electronic conductivity can be at least 1 S/cm, and encapsulant ionic conductivity can be on the order of 1×10−5 S/cm. Higher conductivities of both types generally lead to improved device performance.
In a self-assembled battery system, the encapsulant generally is applied as a thin layer around the anode or cathode particles (typically less than about 5% by volume of any particle), so that the energy density of the battery is not compromised. To enhance the electronic conductivity of the electrode that is formed from the encapsulated active particles, it is generally desirable for the area of inter-particle contact (the “neck”) to be as large as possible. To serve this purpose, it is useful for the encapsulant material to be compliant and able to deform on particle contact, for example, during drying, curing or post-processing of the self-assembled electrochemical device.
Encapsulant materials for active particles in a self-assembled electrochemical device can include at least two components, e.g., a conductive component and a low refractive index component. The components' composition and proportions are varied to provide the desired properties of conductivity and refractive index. It is generally desirable to reduce the lengthscale of separation between the two components (i.e., the size of any phases formed by the two components individually) in the final encapsulant film, to prevent shorting of unlike particles. As the lengthscale of separation is reduced (i.e., there is less phase separation between the conductive and low refractive index components), the composite encapsulant film becomes more homogeneous, such that attractive and repulsive forces exerted by the coated particle surfaces are more homogeneous and uniform. In contrast, when greater lengthscales of phase separation exist in an encapsulant film (e.g., phases with dimensions larger than the distances between the electroactive particles of a system), the resulting heterogeneity at the surface of coated particles can result in attractive forces between unlike particles, thus causing undesirable shorting of the electrochemical device.
Blends of dispersions of a low refractive-index polymer (polytetrafluoroethylene, PTFE) with dispersions of a conductive polymer (Baytron® P, Bayer AG, poly 3,4-ethylenedioxythiophene/polystyenesulfonic acid complex) have been identified as useful materials for encapsulating particles for use in a self-assembled battery system. In such encapsulant materials, the smallest lengthscale of separation between the low refractive index phase and the conductive phase is represented by the dispersed particle size (for PTFE, 10 nm to 100 nm; for Baytron P, about 40 nm). Composite films of such encapsulants are formed on active particles using processes such as spray drying that allow for little control of the encapsulant coating thickness. These encapsulating materials are described generally in co-pending International Published Application WO 03/012908, which is incorporated herein by reference.